I have had great success with the correct size without having to add the extra capacity. The answer to "undersized" units may not be a larger one, because it may be lacking the capacity because of duct problems or other reasons. I don't ever remember seeing a calculation done right and an installation done right to match the calculation being a problem. One of the two have to be wrong, or a combination of minor problems with both.
I was not really addressing a professional that knows what they are doing such as you and TB installing what is right for a specific situation. My point was for the casual person surfing this board that sees a pro recommend extra capacity, then takes that to someone that does not do things right and asks for the extra capacity on top of a system that is probably already too large.
I think we have all seen disgusting jobs (too small, too large, poor design, poor installation, etc.) done by other "professionals" and it makes it hard when we say "call a pro" yet the person asking has been already burned by a "pro". But I hope anyone that reads a few of these posts will be able to learn enough to sort out the good from the bad.